1. Field of the Invention
The present invention relates to integrated circuit manufacturing, and more particularly to insulated-gate field-effect transistors.
2. Description of Related Art
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the channel and the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
In typical IGFET processing, the source and drain are formed by introducing dopants of second conductivity type (P or N) into a semiconductor substrate of first conductivity type (N or P) using a patterned gate as a mask. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide a gate which provides a mask during formation of the source and drain by ion implantation. Thereafter, a drive-in step is applied to repair crystalline damage and to drive-in and activate the implanted dopant.
Complementary metal-oxide semiconductor (CMOS) circuits typically include adjacent N-channel (NMOS) and P-channel (PMOS) devices. Since CMOS inverter circuits use very little power, CMOS is particularly useful in very large-scale integrated (VLSI) circuits where even small power dissipation in each transistor becomes a problem when thousands or millions of transistors are integrated on a chip. CMOS processes typically use N-well and P-well masks early in the processing sequence to define N-type and P-type active regions. CMOS processes also typically include a single masking step for forming gates over the active regions, separate masking steps for implanting lightly doped N-type source/drain regions into the P-type active region and lightly doped P-type source/drain regions into the N-type active region, formation of oxide spacers adjacent to the gates, and then separate masking steps for implanting heavily doped N-type source/drain regions into the P-type active region and heavily doped P-type source/drain regions into the N-type active region.
The diffusion characteristics of various dopants in semiconductor substrates is a complex phenomena which depends, in part, on dopant concentration, substrate (background) doping, junction depth, temperature, and the atmosphere. Unfortunately, at a given high temperature, boron tends to diffuse into the semiconductor substrate at a far greater rate than arsenic or phosphorus. For instance, at 1000.degree. C., boron generally diffuses into silicon at least ten times faster than arsenic or phosphorus. This is partly due to the fact that the activation energy of boron (and consequently the height of the energy barrier that boron must overcome to move within the silicon lattice) is lower than that of arsenic or phosphorus. Unfortunately, the effective channel length of the N-channel device may be significantly greater than that of the P-channel device due to the rapid boron
diffusion. Moreover, it becomes difficult to accurately control the channel junction locations for both the N-channel and P-channel devices.
Accordingly, a need exists for an improved method of making N-channel and P-channel devices in which the channel junction locations are relatively well controlled.